The present invention relates to the cutting of film-like materials.
The solution according to the invention has been developed with particular attention paid to the possible application in automatic packaging systems, for example, for packaging foodstuffs. In particular, the present invention has been developed for possible use in machines for applying tear bands.
The corresponding prior art is extremely extensive, as is demonstrated, for instance, by documents such as IT-B-1 041 468 (to which there corresponds GB-A-1 558 998) and U.S. Pat. No. 3,298,891.
Devices of this basic, common type essentially comprise, with some variations, a ribbon of film-like wrapping material with tear bands applied to the ribbon and oriented transversely with respect to the direction of extension (and of advance) of the ribbon.
The tear bands are obtained starting from a further ribbon of film-like material that is made to advance by steps through a cutting unit. The cutting unit acts in a direction transverse to the direction of feed of the further ribbon of film-like material so as to cut from that ribbon, strips of reduced width, usually selectively variable according to the requirements of use.
The strips of material thus obtained, which are to constitute the tear bands, are then taken up by a transferring device which carries out application of the strips to the wrapping material. This application of the tear bands takes place at given distances corresponding to the dimensions of the products to be wrapped.
The main problem regarding the making of devices of the type described above is linked to the fact that the operation of cutting the tear bands is intrinsically discontinuous since it has to be performed by steps, whereas usually it is desirable that the wrapping material on which the tear bands are applied should be kept continually advancing at a practically constant speed.
The need to reconcile the intermittent operation of the cutting unit that forms the tear bands with the continuous movement of the wrapping material on which the tear bands are applied is more often than not met by adjustments to (according to various modalities) the transferring device. Usually, the transferring device will pick up the tear bands as soon as they have been formed, slowing down or stopping altogether in a position corresponding to the cutting unit, and will then follow a movement of rotation with an acceleration such that, when the tear band, carried by the transferring device, reaches the position in which it is to be transferred onto the wrapping material, it will be advancing at a speed practically corresponding to the speed that the wrapping material is advancing, which is kept continuous and constant.
Consideration of such an arrangement has become increasingly important among the critical factors considered since, according to the by now constant trend in the sector, the speed of operation of the ensemble described (expressed in general in terms of number of tear bands applied per unit time) increases as the rates of operation of the packaging plants increases.
The above-mentioned critical factors also involve the cutting unit, which is frequently built with the use of rotating blades, which may possibly cooperate with counterblades (anvils) carried by the transferring device. A solution of this type is described, for instance, in the Italian patent application for industrial invention TO96A000806.
The above cutting solutions of a dynamic type present, however, the drawback of being difficult to implement, in particular as regards the need to adjust the cutting device exactly and to regulate its operation so that it may be adapted to possible variations in the dimensions of the tear bands and/or in the thickness and nature of the film-like material from which the tear bands are cut.
In various applications that make use of film-like material that is to be cut at pre-set distances, there is already known the solution of resorting to automatic cutting devices which are able to carry out an operation of shearing. These are cutting devices that comprise a blade and a counterblade hinged together like the blades of a pair of scissors or shears.
A solution of the above kind, which is able to ensure good precision in performing the cutting operation, is, however, not applicable, except in very particular cases, to the cutting of tear bands. Usually, the tear bands are made up of very narrow strips which are cut from ribbons of film-like material, and the width of the film-like material defines the length of the tear bands. Since the cutting area ends up being somewhat long, it is necessary to use blades of corresponding extension. Precisely on account of the hinging of the blades in a position corresponding to respective proximal ends, the distal ends of the blades themselves must carry out a somewhat extensive travel, which proves far from compatible with the need to operate at ever-increasing speeds.
The purpose of the present invention is to provide a solution that is able to overcome the drawbacks of the known solutions just described.
According to the present invention, the above purpose is achieved by a cutting process having the characteristics specifically recalled in the ensuing claims. The invention also relates to the corresponding device.